WEATHERAmerica Newsletter, Saturday, July 12, 2008 at 5:00 P.M. CT

WEATHER HAZARDS (During The Next 24 Hours)

SEVERE WEATHER OUTLOOK
(potential for tornadoes, damaging winds, and large hail within the next 24 hours)
Some Thunderstorms May APPROACH Severe Limits
CA....S NV....AZ....S UT....SW CO....NM....W TX

ISOLATED Severe Thunderstorms
(Microbursts, Large Hail, Isolated Tornadoes)
TX Panhandle....OK....AR....S MO....KY....C, E OH....WV....W VA....W MD....W PA
W, C NY....N VT....C, E QB

Some Thunderstorms May APPROACH Severe Limits
MS....AL....FL....W GA....TN


HEAVY RAINFALL OUTLOOK
(potential for an inch or more total rainfall within the next 24 hours)

Scattered Locations In
TX Panhandle....OK....AR....S MO....KY....C, E OH....WV....W VA....W MD....W PA
W, C NY....N VT....C, E QB
(QPF 1 - 3")

Isolated Locations In
MS....AL....FL....W GA....TN
(QPF 1 - 2")

Isolated Locations In
CA....S NV....AZ....S UT....SW CO....NM....W TX
(QPF 1 - 2")


EXCESSIVE HEAT OUTLOOK
(potential for maximum temperature to exceed 95 deg F within the next 24 hours)

Scattered Locations In
CA....C, E OR....SE WA....ID....NV....AZ....UT....WY....C, S MT....W SD....W NE
W KS....OK....TX....NM....CO

Isolated Locations In
S AR....LA....MS....S TN....AL....FL....GA....SC....NC....VA....DC....MD....DE....E PA
NJ....NYC, LI NY

TROPICAL WEATHER SUMMARY
(review of disturbances over the Atlantic Basin and Eastern Equatorial Pacific Ocean)

Eastern Equatorial Pacific Ocean

Tropical Storm Elida is moving west-northwestward, parallel to the shoreline of the Mexican Riviera by about 350 miles. Elida may become a hurricane on Sunday, but is not likely to directly impact any part of Mexico before it breaks up well southwest of Cabo San Lucas, Baja California in about 72 hours.

Other disturbances are seen below Baja California, Nicaragua, and Panama, and any one or all of these systems could bring heavy rainfall impacts to parts of Mexico and Central America through early next week.

Atlantic Basin


Hurricane Bertha is situated 215 miles southeast of Bermuda, moving northward at about 5 mph A general north to northeast motion will allow the Commonwealth to escape the worst part of the storm. Bertha is likely to be picked up by southwest flow ahead of a weak trough approaching the Eastern Seaboard, and may bring heavy rainfall and squalls to the British Isles in about six days.

Other tropical waves are seen off of the coastline of South America. These disturbances are unlikely to strengthen due to vertical wind shear, but are likely to bring considerable thunderstorm activity to the Windward and Leeward Islands, then the Greater Antilles, during the next 120 hours. The ECMWF scheme indicates that the disturbed area may affect much of the Gulf Coast into Texas throughout the medium range.

African ITCZ

A prominent ITCZ-born tropical wave is approaching Cape Verde from Mali and Ghana. This feature has been well advertised on satellite images and numerical model forecasts, and appears poised to become the third named storm of the season after it reaches the central equatorial Atlantic Basin in about three or four days. There is considerable discord among the baroclinic schemes with regard to track and intensity of this feature. But a solid majority of the various outlooks show an impact on the islands rimming the Caribbean Sea, with a U.S. landfall not out of the question at some point during the 11 - 15 day time period.

SHORT RANGE OUTLOOK
(Through The Next 72 Hours)



Modest Cool Intrusion Over Great Lakes, Northeast....

While searing heat is established over much of the nation, the Midwest and Great Lakes are getting the proverbial refuge from the hot air masses. The storm and cold front which is beautifully defined on satellite images will ultimately weaken, but not before lowering humidity and temperature from MN and IA eastward to middle and upper Appalachia over the following 72 hours.

....While Heat Holds Firm Over The Great Plains, Deep South

Despite the passage of the cold front now seen entering the Great Lakes, heat remains a major factor in the forecast for the Great Plains, Dixie, and parts of the Eastern Seaboard. For one thing, the front will weaken as its cold core aloft lifts into eastern Canada. Ridge building will take place with linked Sonoran and Great Smokies signatures, and hot air should rapidly expand early next week from the Front Range into most of the Old South.

Western Monsoon Lowers Temperatures In Desert, Intermountain Regions

Some may be incredulous at saying that temperatures near 90 degrees may count as being unseasonably cool in Phoenix AZ and Las Vegas NV. But with a moisture axis flowing up from the ITCZ over Central America and Mexico into the western states, orographic clouds and precipitation (with a few mesoscale upper disturbances thrown in) have combined to lower readings and increase surface humidity in much of the West. Once a monsoon season gets in full gear, it can take weeks for a standard hot, dry configuration to return. Frankly, I think that some facet of this monsoonal fetch (with lowered afternoon maxima) will be around through the end of the month across the western states.


MEDIUM RANGE OUTLOOK
(Four To Ten Days From Now)

Thunderstorms, Cooler Air The Rule For The Upper Midwest, Great Lakes

Just one look at the ECMWF, GFS, and GGEM model forecasts can tell you that potential for stormy weather will be focused on the Upper Midwest and Great Lakes between July 16 and 22. Note that TWO strong shortwaves are predicted to move eastward through southern Canada, with plentiful energy aloft working against strong surface temperature contrast. Severe thunderstorms and heavy rainfall, followed by quick intrusions of cool temperatures, can be counted on from the Pacific Northwest into W NY and W PA next week.

Tropical Concerns For The Deep South

There are tropical waves lurking over the equatorial Atlantic Basin, and these impulses are predicted to slide west-northwestward over the following ten days, affecting the major islands around the Caribbean Sea before ending up in the Gulf of Mexico. Upper level wind shear (southwesterly flow aloft) will likely prevent intensification of these disturbances. But with a vast supply of moisture and very warm waters nearby, there could be a prominent display of strong thunderstorms and heavy rains across parts of Texas and the lower Mississippi Valley during the medium range.

Extensive Heat Returns To Dixie, Eastern Seaboard

For the period July 16 - 21, the linkage of the Bermuda High and Sonoran heat ridge will allow for a major boost in temperatures from the Front Range communities to the East Coast. The presence of this "mega-ridge" will also allow for westward migration of tropical disturbances from the African ITCZ, one of which may become a named storm (see below).

EXTENDED PERIOD FORECAST
(Between Day 11 And Day 15)

Is That A Cristobal I See In Your Future?

Judging from the baroclinic models and the respective ensemble runs, there is a strong likelihood that the aforementioned ITCZ impulse over western Africa will become a tropical cyclone. There is also a high probability that this feature will take a path further westward than was the case with Hurricane Bertha. Bertha was well detected by the standard equations from about two weeks out, and those models all showed what turned out to be the eventual course of Bertha: a turn to the north long before approach to the U.S.

There are two basic scenarios for the proto-Cristobal (if one assumes that this feature dies become a tropical storm or hurricane). One is that the subtropical high builds further westward across Florida during after July 21, causing a lower latitude path and eventual skirmish with the major islands, before threatening the Sunshine State and eventually the eastern Gulf Coast. The other idea is that a weakness sets up from the Great Lakes to Florida, turning a fully developed hurricane more northward with impacts on the Maritime Provinces (operational GFS) or from the Southeast into much of Appalachia (GFS ensemble package). It is too early to say which course of events will unfold. But it is NOT inconceivable that this disturbance could have important effects on both the Caribbean rim nations and the U.S.

Stay tuned!

Trough Setting Up For Great Lakes, Ohio Valley

In what could be a very important development with regard to the approach of the tropical disturbance mentioned in the above paragraph, the operational GFS shows a fairly prominent upper low over the Great Lakes between July 22 and 26, with a weakness southward across the Appalachian Mountain into Florida. This forecast, if it verified, could allow two important changes in apparent weather: extraordinary rainfall amounts form either thunderstorms or a possible tropical cyclone hit, as well as very cool temperatures across much of the Midwest. Keep in mind that analogues for Agnes (1972) and Heidi (1971) could be in play IF the some of the 12z GFS depictions verified AND a strong tropical cyclone were to develop.

Bermuda High, Sonoran Heat Ridge Slated To Make A Comeback

One of the ongoing, lesser realized themes of this summer so far has been the continual re-appearance of the Bermuda High. This is the reason that the entire Eastern Seaboard has trended warmer than normal, and is partly to blame for excessive rainfall amounts over the Great Lakes and Corn Belt. Most of the numerical models and the variants of these equations show the subtropical high quickly retrogressing and building into the Atlantic Coastal Plain after the passage of the tropical feature. For this reason, above normal temperatures and high relative humidity will likely make a comeback along the Interstate 95 corridor toward the end of July.


Prepared by Meteorologist LARRY COSGROVE on
Saturday, July 12, 2008 at 5:00 P.M. CT

Disclaimer:

The previous statements are my opinions only, and should not be construed as definitive fact. Links provided on this newsletter are not affiliated with WEATHERAmerica and the publisher is not responsible for content posted or associated with those sites.

Copyright 2008 by Larry Cosgrove

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